Studies on the molecular mechanisms of both nucleotide and base excision repair are in progress. The former system involves the use of DNA photoproducts as a model substrate (principally pyrimidine dimers). Enzyme activities and possible non-enzyme cofactors required for the excision of thymine dimers from both DNA and chromatin in vitro are being studied in normal and xeroderma pigmentosum cells in culture. Base excision repair (the removal of uracil from DNA) is being studied in prokaryote cells. A specific DNA glycosidase that removes uracil from DNA has been purified and characterized and subsequent steps in base excision repair are being studied.